Receptor-interacting protein-1 (RIP1) kinase, originally referred to as RIP, is a TKL family serine/threonine protein kinase involved in innate immune signaling. RIP1 kinase is a RHIM domain containing protein, with an N-terminal kinase domain and a C-terminal death domain ((2005) Trends Biochem. Sci. 30, 151-159). The death domain of RIP1 mediates interaction with other death domain containing proteins including Fas and TNFR-1 ((1995) Cell 81 513-523), TRAIL-R1 and TRAIL-R2 ((1997) Immunity 7, 821-830) and TRADD ((1996) Immunity 4, 387-396), while the RHIM domain is crucial for binding other RHIM domain containing proteins such as TRIF ((2004) Nat Immunol. 5, 503-507), DAI ((2009) EMBO Rep. 10, 916-922) and RIP3 ((1999) J. Biol. Chem. 274, 16871-16875); (1999) Curr. Biol. 9, 539-542) and exerts many of its effects through these interactions. RIP1 is a central regulator of cell signaling, and is involved in mediating both pro-survival and programmed cell death pathways which will be discussed below.
The role for RIP1 in cell signaling has been assessed under various conditions [including TLR3 ((2004) Nat Immunol. 5, 503-507), TLR4 ((2005) J. Biol. Chem. 280, 36560-36566), TRAIL (Cell Signal. 2015 February; 27(2):306-14), FAS ((2004) J. Biol. Chem. 279, 7925-7933)], but is best understood in the context of mediating signals downstream of the death receptor TNFR1 ((2003) Cell 114, 181-190). Engagement of the TNFR by TNF leads to its oligomerization, and the recruitment of multiple proteins, including linear K63-linked polyubiquitinated RIP1 ((2006) Mol. Cell 22, 245-257), TRAF2/5 ((2010) J. Mol. Biol. 396, 528-539), TRADD ((2008) Nat. Immunol. 9, 1037-1046) and cIAPs ((2008) Proc. Natl. Acad. Sci. USA. 105, 11778-11783), to the cytoplasmic tail of the receptor. This complex which is dependent on RIP1 as a scaffolding protein (i.e. kinase independent), termed complex I, provides a platform for pro-survival signaling through the activation of the NFκB and MAP kinases pathways ((2010) Sci. Signal. 115, re4). Alternatively, binding of TNF to its receptor under conditions promoting the deubiquitination of RIP1 (by proteins such as A20 and CYLD or inhibition of the cIAPs) results in receptor internalization and the formation of complex II or DISC (death-inducing signaling complex) ((2011) Cell Death Dis. 2, e230). Formation of the DISC, which contains RIP1, TRADD, FADD and caspase 8, results in the activation of caspase 8 and the onset of programmed apoptotic cell death also in a RIP1 kinase independent fashion ((2012) FEBS J 278, 877-887). Apoptosis is largely a quiescent form of cell death, and is involved in routine processes such as development and cellular homeostasis.
Under conditions where the DISC forms and RIP3 is expressed, but apoptosis is inhibited (such as FADD/caspase 8 deletion, caspase inhibition or viral infection), a third RIP1 kinase-dependent possibility exists. RIP3 can now enter this complex, become phosphorylated by RIP1 and initiate a caspase-independent programmed necrotic cell death through the activation of MLKL and PGAM5 ((2012) Cell 148, 213-227); ((2012) Cell 148, 228-243); ((2012) Proc. Natl. Acad. Sci. USA. 109, 5322-5327). As opposed to apoptosis, programmed necrosis (not to be confused with passive necrosis which is not programmed) results in the release of danger associated molecular patterns (DAMPs) from the cell. These DAMPs are capable of providing a “danger signal” to surrounding cells and tissues, eliciting proinflammatory responses including inflammasome activation, cytokine production and cellular recruitment ((2008 Nat. Rev. Immunol 8, 279-289).
Dysregulation of RIP1 kinase-mediated programmed cell death has been linked to various inflammatory diseases, as demonstrated by use of the RIP3 knockout mouse (where RIP1-mediated programmed necrosis is completely blocked) and by Necrostatin-1 (a tool inhibitor of RIP1 kinase activity with poor oral bioavailability). The RIP3 knockout mouse has been shown to be protective in inflammatory bowel disease (including Ulcerative colitis and Crohn's disease) ((2011) Nature 477, 330-334), Psoriasis ((2011) Immunity 35, 572-582), retinal-detachment-induced photoreceptor necrosis ((2010) PNAS 107, 21695-21700), retinitis pigmentosa ((2012) Proc. Natl. Acad. Sci., 109:36, 14598-14603), cerulein-induced acute pancreatitis ((2009) Cell 137, 1100-1111) and Sepsis/systemic inflammatory response syndrome (SIRS) ((2011) Immunity 35, 908-918). Necrostatin-1 has been shown to be effective in alleviating ischemic brain injury ((2005) Nat. Chem. Biol. 1, 112-119), retinal ischemia/reperfusion injury ((2010) J. Neurosci. Res. 88, 1569-1576), Huntington's disease ((2011) Cell Death Dis. 2 e115), renal ischemia reperfusion injury ((2012) Kidney Int. 81, 751-761), cisplatin induced kidney injury ((2012) Ren. Fail. 34, 373-377) and traumatic brain injury ((2012) Neurochem. Res. 37, 1849-1858). Other diseases or disorders regulated at least in part by RIP1-dependent apoptosis, necrosis or cytokine production include hematological and solid organ malignancies ((2013) Genes Dev. 27: 1640-1649), bacterial infections and viral infections ((2014) Cell Host & Microbe 15, 23-35) (including, but not limited to, tuberculosis and influenza ((2013) Cell 153, 1-14)) and Lysosomal storage diseases (particularly, Gaucher Disease, Nature Medicine Advance Online Publication, 19 Jan. 2014, doi:10.1038/nm.3449).
A potent, selective, small molecule inhibitor of RIP1 kinase activity would block RIP1-dependent cellular necrosis and thereby provide a therapeutic benefit in diseases or events associated with DAMPs, cell death, and/or inflammation.